Gisele, bom dia!
Aqui está funcionando normalmente!
install.packages("rrBLUP", dep=T)
require(rrBLUP)
args(A.mat)
# function (X, min.MAF = NULL, max.missing = NULL, impute.method = "mean",
# tol = 0.02, n.core = 1, shrink = FALSE, return.imputed = FALSE)
# NULL